Erythroderma of crucian carp is caused by skin injury and bacterial invasion. The main symptoms are local or most congestion, inflammation and scaling on the body surface of sick fish; In severe cases, the fin root is congested and the fin end is rotten; Sick fish often swim alone on the water and move slowly. This disease can be seen all year round. Prevention and control methods: ① Don't hurt fish when hauling nets. ② Use bleaching powder 1- 1.5g per cubic meter of water, and splash it all over the pool after dissolution. ③ At the same time of external treatment, it is best to take sulfathiazole orally. Every 100kg fish 10g on the first day, and the dosage will be halved from the second day to the sixth day.